To achieve higher performance from turbine engines, manufacturers have designed various parts to operate at higher temperatures. Although these parts are often made from superalloys that can tolerate high temperatures, cooling is critical to reliable operation. Various superalloys, including alloys of Ni and Ti, are well known in the aerospace industry. Turbine blades are one example of superalloy parts that require cooling. Typically, turbine blades have internal cooling passages that permit cooling air to flow though them.
The internal cooling passages in a turbine blade are formed by casting the blade around a ceramic core that can comprise Al.sub.2 O.sub.3, SiO.sub.2, and ZrO.sub.2. The ceramic core is the pattern for the cooling passages. After the blade is cast, the ceramic core is leached out of the blade, leaving a hollow space inside the blade in the form of the cooling passages. The ceramic core is typically leached out of the blade with an aqueous solution of KOH.
Occasionally, parts of the ceramic core are not removed during the leaching step. Because even small parts of the core can block the cooling passages or cause other damage, none of the core can be left inside the turbine blades. Therefore, the blades must be inspected to ensure that the core was completely removed during the leaching step. Currently, turbine blades are inspected with a neutron radiographic method. First, the cooling passages in a blade are bathed in a Gd-containing solution to tag any residual ceramic core. Gd is a strong neutron absorber that highlights any residual ceramic core when exposed to neutrons. If any residual ceramic core is found in the blade, the Gd is washed out of the cooling passages and the leaching step is repeated.
Neutron radiographic inspection has several drawbacks. First, safety precautions required when dealing with a neutron source make the inspection very expensive. Second, only a few facilities are capable of performing the neutron radiographic inspection. If these facilities are not convenient to the blade manufacturing site, the blades must be shipped to the inspection facilities. This increases expenses and delays the manufacturing process. Third, neutron radiation makes the turbine blades slightly radioactive. As a result, the radioactivity in the blades must be allowed to decay to a safe level before further processing. This further delays the manufacturing process. Fourth, the Gd compound can be cumbersome to use. It can streak the walls of the cooling passages, creating false indications. After a false indication, the blade must be cleaned and reinspected. This adds further time and expense to the manufacturing process. The Gd also can interfere with later leaching steps needed to remove residual ceramic core.
Therefore, what is needed in the industry is a method for detecting residual ceramic core in turbine blades and other hollow metal articles that does not require neutron irradiation or Gd for tagging the residual core.